An increasing variety of lighting applications require a precisely controlled spectral characteristic of the radiant energy. It has long been known that combining the light of one color with the light of another color creates a third color. For example, the commonly used primary colors Red, Green and Blue of different amounts can be combined to produce almost any color in the visible spectrum. Adjustment of the amount of each primary color enables adjustment of the spectral properties of the combined light stream. Recent developments for selectable color systems have utilized light emitting diodes or other solid state light sources as the sources of the different light colors.
Light emitting diodes (LEDs) for example were originally developed to provide visible indicators and information displays. For such luminance applications, the LEDs emitted relatively low power. However, in recent years, improved LEDs have become available that produce relatively high intensities of output light. These higher power LEDs, for example, have been used in arrays for traffic lights. Today, LEDs are available in almost any color in the color spectrum.
Additionally, for many lighting applications, an LED based fixture incorporates a circuit board supporting and providing electrical connections to a number of individually packaged LEDs. Often the LEDs are arranged in a fairly tight matrix array (see e.g. U.S. Pat. No. 6,016,038), although a variety of other arrangements are known. For example, U.S. Pat. No. 6,995,355 to Rains, Jr. et al. discloses lighting systems using circular or linear arrangements of LED sets as well as rectangular matrix arrangements and other position patterns. In the noted examples, the sets of LEDs have included LEDs configured for emitting different individual colors or wavelengths (e.g. red, green and blue), although the U.S. Pat. No. 6,995,355 also suggests inclusion of white LEDs or other white light sources. The red, green and blue light allows adjustment and control of the character of the combined light emitted by the system. As the quality of white LEDs continues to improve, newer lights will utilize similar arrangements of LEDs where all the LEDs are white LEDs.
It is well known that many different combinations of wavelengths can produce the same perception of color, and that “Chromaticity” has been long been used to describe the perceived color of a visual stimulus of a human. Many models have been used describe Chromaticity. In one implementation, the CIE system characterizes a given visual stimulus by a luminance parameter Y and two chromaticity coordinates x and y that specify a particular point on the well-known chromaticity diagram. The CIE system parameters Y, x and y are based on the spectral power distribution of the energy emission from a light source. This model also takes into consideration various color sensitivity functions which correlate generally with the response of the human eye.
Also, commonly used primary colors Red, Green and Blue of different amounts can be combined to produce almost any color in the visible spectrum in an optical system. These colors can be represented by the CIE tristimulus values, commonly referred to as X, Y, and Z, respectively, and as illustrated by FIG. 10. Thus, the CIE xyY coordinates may be converted to CIE XYZ coordinates for controlling aforementioned LEDs using the following equations:
                    X        =                              x            y                    ⁢          Y                                    (                  Eqn          .                                          ⁢          A                )                                Y        =        Y                            (                  Eqn          .                                          ⁢          B                )                                Z        =                                            1              -              x              -              y                        y                    ⁢          Y                                    (                  Eqn          .                                          ⁢          C                )            
Conventionally, the aforementioned LED lighting fixtures have been controlled by user inputs using either the xyY or XYZ parameters coordinates above. However, controlling the aforementioned light fixtures in this manner can lead to certain performance inadequacies. For example, LEDs that are capable of emitting light at different wavelengths tend to have different and unique output characteristics. That is, the spectral output differs between similar LEDs based on a given input level that is applied to each LED. For a drive current or power of a given setting, nominally identical LEDs often will produce somewhat different output intensity and may produce light of slightly different colors. Several contributory factors include production variations among the LEDs, as well as differences among the analog drivers that control the flow of current to each LED. As a result, nominally identical light fixtures using LEDs and current of the same types often produce different outputs.
In order to overcome the aforementioned problems, a designer has to manually calibrate each fixture. Specifically, the designer would use a colorimeter to measure an output of each LED (or string of LEDs of a particular color) and manually tune settings for each analog driver circuit until it was decided that the spectral output closely matched the desired color according to the input setting. Then the analog driver settings would be saved as a preset. This process would be repeated for each color of LED used in the light fixture, until all of the LEDs in a given fixture had been manually tuned and presets had been established to support a number of overall color output settings.
It is evident that conventional calibration techniques take time and cost money especially when manufacturing LED fixtures on a large scale. Also, operations of systems calibrated in such a manner are limited by the practical number of presets available. Hence, a need exists for a way to calibrate LED fixtures in an efficient manner that can be implemented on a large production scale. Preferably, such a technique should offer an increased degree of responsiveness to user inputs, without the need for storing large numbers of preset values.